Such electromechanical converters are known, inter alia from the international patent application WO 00/34066, from U.S. patent specification U.S. Pat. No. 3,683,249 and from the European patent application EP-A-0 866 544. They can be applied in the form of an electric variable transmission (EVT), not only in a variety of road vehicles, in particular local buses, but also in rail vehicles such as diesel trains, and in vessels.
More particularly, WO 00/34066 describes a hybrid drive with an electromechanical transmission, comprising a stator which surrounds two concentrically arranged rotors, while the rotors are mechanically coupled to an IC engine and a drive train, respectively. The outer rotor, which is arranged between the inner rotor and the stator, is referred to as interrotor in the context of this application. Rotor-interrotor and interrotor-stator in WO 00/34066 form two mechanically integrated electric machines that can both be of either the synchronous or the asynchronous type. The flux transfer between both the rotor-interrotor and the interrotor-stator is radial or axial. The electric machines are electromagnetically separated: there is no flux transfer from the rotor via the interrotor to the stator or vice versa. The interrotor is built up from mechanically coupled, but electromagnetically separated parts. The electric machines of the electromechanical transmission are each controlled with individual control means.
The electric variable transmission (EVT) is an electromechanical converter with two mechanical ports, viz. a primary (driving) shaft and a secondary (driven) shaft, and an electric gate via which energy can be exchanged. If the electric gate is not used, the EVT works as a common infinitely variable transmission, where the transfer ratio has a very wide range. The EVT fulfills a function that is comparable to that of a combination of a clutch and a gearbox of a vehicle. In combination with the EVT, the combustion engine in fact works as a power source, whereby the speed can be set according to an optimum characteristic of the engine. The fuel consumption, the noise level and the emission of noxious gases from the vehicle can thus be reduced. Because the EVT is an infinitely variable transmission, there are no gearshifting shocks during acceleration. It also means that always acceleration with a constant (maximally permissible) power is possible, and not, as in a conventional gearbox, with a more or less sawtooth-shaped course of the power as a function of the time. With the same engine, the acceleration with an EVT therefore proceeds faster than with a conventional gearbox.
The EVT can work as a substantially wear-free starter motor by connecting the electric gate to an on-board battery. As a consequence, local buses equipped with an EVT, can, for instance, switch off their engine at bus stops without any objection, which is more comfortable and moreover leads to a substantial saving of fuel, since frequent starting entails undue wear of the starter motor and the starter ring in conventional starter motors.
Via the electric gate, via a power electronic converter, the on-board network can be supplied and hence the on-board battery can be charged. Thus, a substantially wear-free dynamo has been obtained. Since conventional dynamos have a poor efficiency, this also leads to a minor saving of fuel. In an electric system with a common dynamo, the power is practically limited by the belt drive and the low on-board voltage. In the use of the EVT, via power electronic converters, higher voltage levels can be simply created and the power is limited only by the diesel engine. This means that some auxiliary provisions that are now driven directly by the engine, such as, for instance, the pump for the power steering, or the compressor in buses, can be driven electrically with a high efficiency. They can then be switched on and off at will, so that the no-load losses are less. This also leads to fuel saving.
With the EVT, it is simply possible to brake by way of the motor. The braking power can then be increased by increasing the speed of the engine. However, this is accompanied by an increased noise production. In addition, it is possible to dissipate the brake energy in resistances that are connected to the electric gate. This makes it possible to brake to a halt, this in contrast, for instance, to a conventional retarder in buses. When braking, possibly, the motor can even be switched off, so that the fuel consumption is reduced and the engine does not produce any noise.
To the electric gate, a buffer in the form of a battery or a flywheel system can be connected. With these, brake energy can be stored, which can subsequently be used again for acceleration. This relatively costly extension can, particularly with local buses, yield a substantial saving of fuel.
The electric gate makes the converter particularly suitable for use in hybrid vehicles; the mechanical energy for the cardan shaft can be produced both by the IC engine and by an electric source.
The most important disadvantage of the existing EVT is its mass. It is significantly greater than that of a conventional gearbox. In addition, the existing EVT is also more expensive than a conventional gearbox.
As has already been mentioned, an electromechanical converter, as is described in the opening paragraph, is known from the European patent EP-A-0 866 544. The electromechanical converter described therein, however, has as a drawback that the interrotor is formed by two rotor parts which are mechanically connected with each other, but are electromagnetically separated from each other, so that the rotor with the one rotor part forms a first machine and the other rotor part with the stator forms a second machine working independently of the first. Although as a result the advantage is obtained that the magnetic flux in the two machines can be separately controlled, it is a serious drawback that as a result of this structure the electromechanical converter is large and heavy.